Power Controllers and Power Converters with Configurable Feedback Loop for Different Nominal Output Voltages

ABSTRACT

A power controller is in use of a power converter whose output voltage can be regulated at a first nominal output voltage or a second nominal output voltage less than the first nominal output voltage. An ON-time controller controls an ON time of a driving signal provided to a power switch according to a compensation signal. A frequency controller controls, based on the compensation signal and a feedback signal, a switching frequency of the driving signal. If the compensation signal has an input waveform and when the output voltage is regulated at the first or second nominal output voltage, the frequency controller provides first or second settling time to stabilize the switching frequency, respectively. The second settling time is longer than the first settling time.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of pending U.S. application Ser. No. 16/026,299, filed on Jul. 3, 2018, which claims priority to and the benefit of Taiwan Application Series Number 106124926 filed on Jul. 25, 2017, which is incorporated by reference in its entirety.

BACKGROUND

The present disclosure relates generally to switching mode power supplies (SMPSs), more particularly to SMPSs and related control methods capable of providing different control feedback loops for different nominal output voltages.

Universal Serial Bus (USB) is one of the communication interfaces most broadly used in daily life nowadays. Beside its reliable, rapid data transmission, USB also plays as an interface delivering limited power to the peripherals connected to it. Most mobile phones are charged using USB chargers, for example.

To make USB more suitable for powering various electric apparatuses and reducing the number of power cables needed, USB Implementers Forum Inc., a non-profit corporation founded by the group of companies that developed the USB specification, has announced USB Power Delivery (PD) to enable the maximum functionality of USB by providing more flexible power delivery along with data over a single cable. USB PD offers increased power levels from existing standards up to 100 W, so it is possible to enable new higher power use cases such as USB powered hard disk drivers and printers.

USB PD requires a USB charger having its output voltage variable in a range from 5V to 20V, and this range could be expanded as broad as being from 3V to 20V in the future. So far, the nominal output voltages of a USB charger complying USB PD are 5V, 12V and 20V, meaning the USB charger should regulate its output voltage at 5V, 12V or 20V. A USB charger complying with USB PD for example might regulate its output voltage at 20V, and switch to regulate its output voltage at 5V upon a demand received from a connected, charged apparatus.

The feedback loop for regulating an output voltage at 5V may be inappropriate for the feedback loop regulating the output voltage at 20V, however. To optimize the output regulation of a USB charger, it is expected to have the feedback loop configurable or changeable when the nominal output voltage of the USB charger is switched.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following drawings. In the drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified. These drawings are not necessarily drawn to scale. Likewise, the relative sizes of elements illustrated by the drawings may differ from the relative sizes depicted.

The invention can be more fully understood by the subsequent detailed description and examples with references made to the accompanying drawings, wherein:

FIG. 1, according to embodiments of the invention, demonstrates a power converter with a flyback topology, capable of being a USB charger to charge a rechargeable apparatus;

FIG. 2 demonstrates a power controller capable of replacing the power controller in FIG. 1 according to embodiments of the invention;

FIGS. 3A, 3B, 3C and 3D demonstrates four low-pass filters;

FIG. 4 shows frequency curve CV_(f);

FIG. 5A shows that compensation signal V_(COMP) jumps up abruptly from V_(COMP1) to V_(COMP2) at moment t_(STEP);

FIGS. 5B and 5C show the step responses of switching frequency f_(SW) when output voltage V_(OUT) is regulated at 20V and 5V respectively;

FIG. 6 demonstrates a power controller capable of replacing the power controller in FIG. 1 according to embodiments of the invention; and

FIG. 7 shows frequency curves CV_(f-5V) and CV_(f-20V).

DETAILED DESCRIPTION

A USB charger is used as an embodiment of the invention, but the invention is not limited to. Embodiments of the invention include other kinds of switching mode power supplies, and the disclosure of this invention is not on purpose to limit the scope of the invention.

FIG. 1, according to embodiments of the invention, demonstrates a power converter 10 with a flyback topology, capable of being a USB charger to charge rechargeable apparatus 13. Bridge rectifier 11 rectifies alternating-current (AC) voltage V_(AC) to provide an input voltage V_(IN) and an input ground voltage, which power converter 10 converts to output voltage V_(OUT) and an output ground voltage. Rechargeable apparatus 13 sends selection signal S_(SEL), based on which power converter 10 regulates output voltage V_(OUT) at one of two or more nominal output voltages. In other words, the nominal output voltage of power converter 10 is configurable, determined by selection signal S_(SEL). In this following specification, two nominal output voltages are, but are not limited to be, 20V and 5V respectively.

Based on selection signal S_(SEL), reference voltage generator 16 provides reference voltage V_(REF1), with which comparator 18 compares output voltage V_(OUT) to produce compensation voltage V_(COMP) at compensation node COMP via photo coupler 20, so as to provide feedback control to power controller 12 and to regulate output voltage V_(OUT) at either 5V or 20V as selection signal S_(SEL) selects.

Power converter 10 has a transformer with primary winding PRM, secondary winding SEC and auxiliary winding AUX, inductively coupled to each other. Power controller 12 generates driving signal S_(DRV), based on compensation signal V_(COMP) at compensation node COMP, to turn ON or OFF power switch 14, which accordingly conducts or stops inductor current I_(PRM) flowing through primary winding PRM. Power controller 12 has feedback node FB connected via resistors RA1 and RA2 to auxiliary winding AUX. Feedback signal V_(FB) at feedback node FB, under some circumstances, represents the voltage drop across auxiliary winding AUX.

According to an embodiment of the invention, power controller 12 controls switching frequency f_(SW) of driving signal S_(DRV) based on compensation signal V_(COMP) and feedback signal V_(FB). The relationship between compensation signal V_(COMP) and switching frequency f_(SW) can be represented by a frequency curve demonstrated in a V_(COMP)-to-f_(SW) chart. When compensation signal V_(COMP) becomes less than a predetermined fold voltage V_(FOLD), the frequency curve in the V_(COMP)-to-f_(SW) chart indicates that switching frequency f_(SW) reduces according to a frequency-reduction slope SL. Power controller 12 at the same time detects output voltage V_(OUT) of power converter 10 from feedback signal V_(FB) to determine whether the present nominal output voltage is 20V or 5V. If output voltage V_(OUT) is determined to be about 20V, power controller 12 determines switching frequency f_(SW) directly based on compensation signal V_(COMP) and the frequency curve. If output voltage V_(OUT) is determined to be about 5V however, compensation signal V_(COMP) is additionally low-pass filtered before being forwarded to determine switching frequency f_(SW). For a steady state, the frequency curve is the same regardless of whether the nominal output voltage is 5V or 20V. Nevertheless, if compensation signal V_(COMP) varies to have an input waveform, a step input for example, the settling time for switching frequency f_(SW) being stabilized when nominal output voltage is 5V will be longer than that when nominal output voltage is 20V.

According to another embodiment of the invention, power controller 12 controls switching frequency f_(SW) of driving signal S_(DRV) based on compensation signal V_(COMP) and feedback signal V_(FB). The relationship between compensation signal V_(COMP) and switching frequency f_(SW) can be represented by a frequency curve demonstrated in a V_(COMP)-to-f_(SW) chart. When compensation signal V_(COMP) becomes less than a predetermined fold voltage V_(FOLD), the frequency curve in the V_(COMP)-to-f_(SW) chart indicates that switching frequency f_(SW) reduces according to a frequency-reduction slope SL. Power controller 12 at the same time detects output voltage V_(OUT) of power converter 10 from feedback signal V_(FB) to determine whether the present nominal output voltage is 20V or 5V. If output voltage V_(OUT) is determined to be about 20V, meaning the nominal output voltage is 20V, frequency-reduction slope SL has a first drop-off rate; and if output voltage V_(OUT) is determined to be about 5V, frequency-reduction slope SL has a second drop-off rate less than the first drop-off rate. In one embodiment of the invention, the predetermined fold voltage V_(FOLD) is a constant, unchanged even if output voltage V_(OUT) varies due to the change of the nominal output voltage.

FIG. 2 demonstrates a power controller 12 a capable of replacing power controller 12 in FIG. 1 according to embodiments of the invention.

Power controller 12 a includes a switch driver 38 a, an ON-time controller 31 a, a frequency controller 30 a and a SR flip-flop 36 a.

Switch driver 38 a amplifies PWM signal S_(PWM) to become driving signal S_(DRV) with suitable voltage and current that drives power switch 14 in FIG. 1. PWM signal S_(PWM) substantially equals to driving signal S_(DRV) in view of their logic values, and they might be different in logic voltage levels.

ON-time controller 31 a controls an ON time of driving signal S_(DRV) according to compensation signal V_(COMP), and has an attenuator 32 a and a comparator 34 a. Attenuator 32 a generates output V_(COMP-SCL) by attenuating compensation signal V_(COMP). For example, attenuator 32 a might include a voltage-divider to attenuate compensation signal V_(COMP). When current-sense signal V_(CS) at current-sense node CS exceeds output V_(COMP-SCL), comparator 34 a resets SR flip-flop 36 a, making PWM signal S_(PWM) having a logic value of “0” and ending ON time T_(ON) of power switch 14.

Frequency controller 30 a, based upon compensation signal V_(COMP) and feedback signal V_(FB), provides clock signal S_(CLK) to periodically set SR flip-flop 36 a, making PWM signal S_(PWM) have a logic value of “1” and starting ON time T_(ON) of power switch 14. Frequency controller 30 a includes an output voltage detector 41 a, a low-pass filter 40 a and a frequency generator 42 a.

Output voltage detector 41 a, based on the timing provided by PWM signal S_(PWM), samples feedback signal V_(FB) and compares the sample result with reference voltage V_(REF2), so as to roughly know whether output voltage V_(OUT) is 20V or 5V. For example, if output voltage V_(OUT) is about 20V, the sample result is configured to be higher than reference voltage V_(REF2), so signal S_(LV) from output voltage detector 41 a has logic value of “0”, and the present nominal voltage is expected to be 20V. If output voltage V_(OUT) is about 5V, the sample result is configured to be less than reference voltage V_(REF2), so signal S_(LV) has logic value of “1”, and the present nominal voltage is expected to be 5V.

The filtering function of low-pass filter 40 a is configurable, based on the logic value of signal S_(LV). Output voltage detector 41 a can dis-enable or enable the filtering function of low-pass filter 40 a. For example, if signal S_(LV) is “1” in logic, low-pass filter 40 a low-pass filters compensation signal V_(COMP) to provide delayed signal V_(COMP-LP). In the opposite, if the signal S_(LV) is “0” in logic, low-pass filter 40 a stops low-pass filtering, and passes compensation signal V_(COMP) substantially without delay, such that delayed signal V_(COMP-LP) is about equal to compensation signal V_(COMP). FIGS. 3A, 3B, 3C and 3D demonstrates low-pass filters 40 aa, 40 ab, 40 ac and 40 ad, each of which could embody the low-pass filter 40 a according to the invention. Each of low-pass filters 40 aa and 40 ac performs low-pass filtering by using a resistor-capacitor circuit, and each of low-pass filters 40 ab and 40 ad does by using a switched capacitor circuit. In each of low-pass filters 40 aa and 40 ab, signal S_(LV) controls a bypass switch SW_(P), which, when being turned ON, directly makes compensation signal V_(COMP) delayed signal V_(COMP-LP), and disables the function of low-pass filtering. Analogously, in each of low-pass filters 40 ac and 40 ad, signal S_(LV) controls an isolation switch SW_(ISO), which, when being turned OFF, separates a filtering capacitor from the signal path in the respective low-pass filter, so as to disables the function of low-pass filtering.

Low-pass filter 40 a according to embodiments of the invention is not limited to have no function of low-pass filtering when the signal S_(LV) is “0”. For example, when the signal S_(LV) is “0” low-pass filter 40 a could be a low-pass filter weaker than low-pass filter 40 a could be when the signal S_(LV) is “1”. Preferably, the direct current response of low-pass filter 40 a does not change if signal S_(LV) toggles its logic value, but a high-frequency response of low-pass filter 40 a weakens when signal S_(LV) switches from logic “0” to logic “1”.

Frequency generator 42 a in FIG. 2 provides clock signal S_(CLK) according to delayed signal V_(COMP-LP). Clock signal S_(CLK) substantially determines the moment when power switch 14 is turned ON, so as to decide switching frequency f_(SW) of PWM signal S_(PWM) and driving signal S_(DRV). FIG. 4 shows frequency curve CV_(f), which demonstrates the relationship between delayed signal V_(COMP-LP) and switching frequency f_(SW) that frequency generator 42 a provides. As shown in FIG. 4, frequency generator 42 a makes switching frequency f_(SW) about a constant maximum frequency f_(MAX) when delayed signal V_(COMP-LP) exceeds fold voltage V_(FOLD). When delayed signal V_(COMP-LP) decreases to be less than fold voltage V_(FOLD), switching frequency f_(SW) reduces according to a frequency-reduction slope SL, the tilted slope of frequency curve CV_(f) between fold voltage V_(FOLD) and light-load voltage V_(L) in FIG. 4. If delayed signal V_(COMP-LP) becomes less than light-load voltage V_(L), switching frequency f_(SW) remains at about a constant minimum frequency f_(MIN).

FIG. 5A shows that compensation signal V_(COMP) jumps up abruptly from V_(COMP1) to V_(COMP2) at moment t_(STEP) and has an input waveform about representing a step input. FIGS. 5B and 5C show the step responses of switching frequency f_(SW) when output voltage V_(OUT) is regulated at 20V and 5V respectively. Shown in FIG. 5B where output voltage V_(OUT) is about 20V, it, in response to the step input of compensation signal V_(COMP) in FIG. 5A, costs settling time T_(SETTLE1) for frequency controller 30 a to stabilize switching frequency f_(SW), which begins from first frequency f₁ and finally stabilizes at second frequency f₂. Shown in FIG. 5C where output voltage V_(OUT) is about 5V, switching frequency f_(SW), in response to the step input of compensation signal V_(COMP) in FIG. 5A, varies from first frequency f₁ and finally stabilizes at second frequency f₂. Settling time T_(SETTLE2) in FIG. 5C is longer than settling time T_(SETTLE1) in FIG. 5B, nevertheless. As detailed before, low-pass filter 40 a in FIG. 2 is enabled when output voltage V_(OUT) is about 5V, and dis-enabled when output voltage V_(OUT) is 20V. Therefore, the change in compensation signal V_(COMP) needs longer signal propagation delay to actually affect frequency generator 42 a when output voltage V_(OUT) is about 5V than it does when output voltage V_(OUT) is about 20V. Therefore, settling time T_(SETTLE2) is longer than settling time T_(SETTLE1) as shown in FIGS. 5A, 5B and 5C.

FIGS. 5A, 5B and 5C also show that in response to the step input of compensation signal V_(COMP) in FIG. 5A, switching frequency f_(SW) stabilizes finally at second frequency f₂ no matter whether output voltage V_(OUT) is regulated at 5V or 20V.

The input waveform of compensation signal V_(COMP) is not limited to be a step input, however. For example, compensation signal V_(COMP) might have an input waveform representing a unit pulse. In response to that unit pulse, switching frequency f_(SW) drifts away from an original frequency and, after a settling time, comes back to and settles at the original frequency. The settling time needed when nominal output voltage is 5V is longer than that needed when nominal output voltage is 20V, because longer signal propagation delay is needed when nominal output voltage is 5V.

The low-pass filtering provided when output voltage V_(OUT) is about 5V slows the response of switching frequency f_(SW) to the change in compensation signal V_(COMP), and therefore possibly stabilizes the feedback control more.

FIG. 6 demonstrates a power controller 12 b capable of replacing power controller 12 in FIG. 1 according to embodiments of the invention. The same or similar components commonly shared by power controllers 12 b and 12 a can be understood in light of the aforementioned teaching regarding to power controller 12 a and will not be detailed redundantly for brevity.

Power controller 12 b, unlike power controller 12 a, has frequency controller 30 b with output voltage detector 41 a and frequency generator 42 b.

Output voltage detector 41 a, based on the timing provided by PWM signal S_(PWM), detects output voltage V_(OUT) via feedback node FB and auxiliary winding AUX, so as to roughly know whether output voltage V_(OUT) is 20V or 5V. For example, if output voltage V_(OUT) is about 20V, signal S_(LV) has logic value of “0”, and the present nominal voltage is expected to be 20V. If output voltage V_(OUT) is about 5V, signal S_(LV) has logic value of “1”, and the present nominal voltage is expected to be 5V.

Frequency generator 42 b provides clock signal S_(CLK) according to compensation signal V_(COMP) and signal S_(LV). Clock signal S_(CLK) substantially determines the moment when power switch 14 is turned ON, so as to decide switching frequency f_(SW) of PWM signal S_(PWM) and driving signal S_(DRV). FIG. 7 shows frequency curves CV_(f-5V) and CV_(f-20V), different relationships between compensation signal V_(COMP) and switching frequency f_(SW) that frequency generator 42 b provides. Frequency generator 42 b employs frequency curve CV_(f-5V) when output voltage V_(OUT) is about 5V, and frequency curve CV_(f-20V) when output voltage V_(OUT) is about 20V. Take frequency curve CV_(f-20V) as an example, frequency generator 42 b makes switching frequency f_(SW) about a constant maximum frequency f_(MAX) when compensation signal V_(COMP) exceeds fold voltage V_(FOLD). When compensation signal V_(COMP) decreases to be less than fold voltage V_(FOLD), switching frequency f_(SW) reduces according to a frequency-reduction slope SL_(20V), the tilted slope of frequency curve CV_(f-20V) between fold voltage V_(FOLD) and light-load voltage V_(L-20V) in FIG. 7. If compensation signal V_(COMP) becomes less than light-load voltage V_(L-20V), switching frequency f_(SW) remains at about a constant minimum frequency f_(MIN). Frequency curve CV_(f-5V) in FIG. 7, unlike frequency curve CV_(f-20V), reduces switching frequency f_(SW) according to a frequency-reduction slope SL_(5V), the tilted slope of frequency curve CV_(f-5V) between fold voltage V_(FOLD) and light-load voltage V_(L-5V), while, as shown in FIG. 7, frequency-reduction slope SL_(5V) has a drop-off rate less than frequency-reduction slope SL_(20V) does. Frequency-reduction slopes SL_(20V) and SL_(5V) commonly share fold voltage V_(FOLD), and light-load voltage V_(L-5V) is less than light-load voltage V_(L-20V).

In other words, output voltage detector 41 a makes frequency generator 42 b respond to compensation signal V_(COMP) to synthesize switching frequency f_(SW) based on frequency-reduction slop SL_(20V) when output voltage V_(OUT) is regulated at about 20V, and based on frequency-reduction slop SL_(5V) when output voltage V_(OUT) is regulated at about 5V, where frequency-reduction slop SL_(5V), in comparison with frequency-reduction slop SL_(20V), has a less drop-off rate.

As frequency-reduction slop SL_(5V) is less tilted than frequency-reduction slop SL_(20V), switching frequency f_(SW) could less respond to the change in compensation signal V_(COMP) when output voltage V_(OUT) is about 5V than it does when output voltage V_(OUT) is about 20V, to form an adjustable control loop fitting different nominal output voltages.

While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. 

What is claimed is:
 1. A power controller for a power converter converting an input voltage into an output voltage, wherein the power converter is configured to, based on a selection signal, switch the output voltage from a first nominal output voltage to a second nominal output voltage less than the first nominal output voltage, the power converter includes a primary winding, a secondary winding and an auxiliary winding, the power controller comprising: a switch driver for providing a driving signal to a power switch to control an inductor current through the primary winding; and an ON-time controller for controlling an ON time of the driving signal according to a compensation signal; and a frequency controller for, based on the compensation signal and a feedback signal, controlling a switching frequency of the driving signal, wherein, when the output voltage is regulated at the first nominal output voltage and the compensation signal has an input waveform, the frequency controller provides a first settling time to stabilize the switching frequency, when the output voltage is regulated at the second nominal output voltage and the compensation signal has the input waveform, the frequency controller provides a second settling time to stabilize the switching frequency, and the second settling time is longer than the first settling time; wherein the compensation signal is generated by comparing the output voltage with a reference voltage determined by the selection signal; and the feedback signal is at a feedback node coupled to the auxiliary winding.
 2. The power controller as claimed in claim 1, wherein the frequency controller comprises: a low-pass filter for low-pass filtering the compensation signal to generate a delayed compensation signal; wherein when the output voltage is regulated at the first nominal output voltage the low-pass filter is dis-enabled, not providing the low-pass filtering; and when the output voltage is regulated at the second nominal output voltage the low-pass filter is enabled, providing the low-pass filtering.
 3. The power controller as claimed in claim 2, wherein the frequency controller comprises: an output voltage detector coupled to the feedback node, for controlling the low-pass filter based on the feedback signal; when the output voltage is regulated at the first nominal output voltage the output voltage detector dis-enables the low-pass filter; and when the output voltage is regulated at the second nominal output voltage the output voltage detector enables the low-pass filter.
 4. The power controller as claimed in claim 2, wherein the low-pass filter comprises a switched capacitor circuit.
 5. The power controller as claimed in claim 1, wherein when the compensation signal becomes lower than a predetermined voltage the frequency controller reduces the switching frequency.
 6. The power controller as claimed in claim 1, wherein the input waveform represents a step input.
 7. A power converter for converting an input voltage into an output voltage, and, based on a selection signal, being capable of switching the output voltage from a first nominal output voltage to a second nominal output voltage less than the first nominal output voltage, the power converter comprising: a transformer with a primary winding, a secondary winding and an auxiliary winding, inductively-coupled to one another; a power switch for controlling an inductor current through the primary winding; a power controller, for, based on a compensation signal and a feedback signal, providing a driving signal to the power switch, the power controller comprising: a feedback node, coupled to the auxiliary winding to provide the feedback signal; an ON-time controller for controlling an ON time of the driving signal according to the compensation signal; and a frequency controller for, based on the compensation signal and the feedback signal, controlling a switching frequency of the driving signal, wherein, when the output voltage is regulated at the first nominal output voltage and the compensation signal has an input waveform, the frequency controller provides a first settling time to stabilize the switching frequency, when the output voltage is regulated at the second nominal output voltage and the compensation signal has the input waveform, the frequency controller provides a second settling time to stabilize the switching frequency, and the second settling time is longer than the first settling time; wherein the compensation signal is generated by comparing the output voltage with a reference voltage determined by the selection signal.
 8. The power converter as claimed in claim 6, wherein the frequency controller comprises: a low-pass filter for low-pass filtering the compensation signal to generate a delayed compensation signal; wherein when the output voltage is regulated at the first nominal output voltage the low-pass filter is dis-enabled, not providing the low-pass filtering; and when the output voltage is regulated at the second nominal output voltage the low-pass filter is enabled, providing the low-pass filtering.
 9. The power converter as claimed in claim 8, wherein the frequency controller comprises: an output voltage detector coupled to the feedback node, for controlling the low-pass filter based on the feedback signal; when the output voltage is regulated at the first nominal output voltage the output voltage detector dis-enables the low-pass filter; and when the output voltage is regulated at the second nominal output voltage the output voltage detector enables the low-pass filter.
 10. The power converter as claimed in claim 8, wherein the low-pass filter comprises a resistor-capacitor circuit.
 11. The power converter as claimed in claim 8, wherein the low-pass filter comprises a switched capacitor circuit.
 12. The power converter as claimed in claim 8, wherein the input waveform represents a step input.
 13. The power converter as claimed in claim 7, wherein the frequency controller, in response to the input waveform of the compensation signal, stabilizes the switching frequency at a common frequency no matter whether the output voltage is regulated at the first or the second nominal output voltage.
 14. A power controller for a power converter converting an input voltage into an output voltage, wherein the power converter includes a primary winding, a secondary winding and an auxiliary winding inductively coupled to each other, the power controller comprising: a switch driver for providing a driving signal to a power switch to control an inductor current through the primary winding; and an ON-time controller for controlling an ON time of the driving signal according to a compensation signal, wherein the compensation signal is generated by comparing the output voltage with a first reference voltage; and a frequency controller for controlling a switching frequency of the driving signal based on the compensation signal and a feedback signal at a feedback node coupled to the auxiliary winding, the frequency controller comprising: a low-pass filter for low-pass filtering the compensation signal to generate a delayed compensation signal; a frequency generator for determining the switching frequency according to the delayed compensation signal; and an output voltage detector, for comparing the feedback signal with a second reference voltage, to control the low-pass filter.
 15. The power controller as claimed in claim 14, wherein the output voltage detector disables the low-pass filter when the power converter regulates the output voltage at a first nominal output voltage, and enables the low-pass filter when the power converter regulates the output voltage at a second nominal output voltage less than the first nominal output voltage.
 16. The power controller as claimed in claim 15, wherein the output voltage detector samples the feedback voltage to generate a sample result, and compares the sample result with the second reference voltage.
 17. The power controller as claimed in claim 14, wherein the low-pass filter comprises a resistor, a capacitor and a switch controlled by the output voltage detector.
 18. The power controller as claimed in claim 17, wherein the switch is connected in parallel with the resistor.
 19. The power controller as claimed in claim 17, wherein the switch is connected in series between the resistor and the capacitor.
 20. The power controller as claimed in claim 17, wherein the resistor is in form of a switched capacitor circuit. 